where fn is a function symbol of arity 2, event-name, if supplied,
is a symbol, and all other arguments are as specified in the
documentation for defthm. The defequiv macro expands into a call
of defthm. The name of the defthm is fn-is-an-equivalence, unless
event-name is supplied, in which case event-name is the name used.
The term generated for the defthm event states that fn is Boolean,
reflexive, symmetric, and transitive. The rule-class
:equivalence is added to the rule-classes specified, if it is not
already there. All other arguments to the generated defthm form
are as specified by the other keyword arguments above.